Revert "Revert "Allow validators to reset to the slot which matches their last voted slot (backport solana-labs#28172) (solana-labs#28187)""

This reverts commit 2e25fbc.

Author: carllin

core/src/heaviest_subtree_fork_choice.rs

@@ -940,18 +940,6 @@ impl HeaviestSubtreeForkChoice {
                         }
                     }
                 }
-                let heaviest_slot_hash_on_same_voted_fork =
-                    heaviest_slot_hash_on_same_voted_fork.unwrap();
-
-                if heaviest_slot_hash_on_same_voted_fork == last_voted_slot_hash {
-                    info!(
-                        "heaviest_slot_on_same_voted_fork: {:?}, last_voted_slot_hash: {:?}",
-                        heaviest_slot_hash_on_same_voted_fork, last_voted_slot_hash
-                    );
-                    None
-                } else {
-                    Some(heaviest_slot_hash_on_same_voted_fork)
-                }
             })
     }
 
@@ -3035,9 +3023,10 @@ mod test {
 
         // After marking the last vote in the tower as invalid, `heaviest_slot_on_same_voted_fork()`
         // should disregard all descendants of that invalid vote
-        assert!(heaviest_subtree_fork_choice
-            .heaviest_slot_on_same_voted_fork(&tower)
-            .is_none());
+        assert_eq!(
+            heaviest_subtree_fork_choice.heaviest_slot_on_same_voted_fork(&tower),
+            None
+        );
 
         // Adding another descendant to the invalid candidate won't
         // update the best slot, even if it contains votes

core/src/replay_stage.rs

@@ -2980,9 +2980,18 @@ impl ReplayStage {
         );
     }
 
-    // Given a heaviest bank, `heaviest_bank` and the next votable bank
-    // `heaviest_bank_on_same_voted_fork` as the validator's last vote, return
-    // a bank to vote on, a bank to reset to,
+    /// Given a `heaviest_bank` and a `heaviest_bank_on_same_voted_fork`, return
+    /// a bank to vote on, a bank to reset to, and a list of switch failure
+    /// reasons.
+    ///
+    /// If `heaviest_bank_on_same_voted_fork` is `None` due to that fork no
+    /// longer being valid to vote on, it's possible that a validator will not
+    /// be able to reset away from the invalid fork that they last voted on. To
+    /// resolve this scenario, validators need to wait until they can create a
+    /// switch proof for another fork or until the invalid fork is be marked
+    /// valid again if it was confirmed by the cluster.
+    /// Until this is resolved, leaders will build each of their
+    /// blocks from the last reset bank on the invalid fork.
     pub fn select_vote_and_reset_forks(
         heaviest_bank: &Arc<Bank>,
         // Should only be None if there was no previous vote
@@ -5925,7 +5934,7 @@ pub(crate) mod tests {
             &mut vote_simulator.latest_validator_votes_for_frozen_banks,
         );
         assert!(vote_fork.is_none());
-        assert_eq!(reset_fork.unwrap(), 3);
+        assert_eq!(reset_fork, Some(3));
 
         // Now mark 2, an ancestor of 4, as duplicate
         blockstore.store_duplicate_slot(2, vec![], vec![]).unwrap();
@@ -7026,7 +7035,6 @@ pub(crate) mod tests {
         );
         let (heaviest_bank, heaviest_bank_on_same_fork) = heaviest_subtree_fork_choice
             .select_forks(&frozen_banks, tower, progress, ancestors, bank_forks);
-        assert!(heaviest_bank_on_same_fork.is_none());
         let SelectVoteAndResetForkResult {
             vote_bank,
             reset_bank,